This invention relates to a wire bonding method and apparatus and more particularly to a method and apparatus for bonding wire leads to a semiconductor device.
Many forms of circuits employ semiconductors mounted on circuit boards. Semiconductor device production processes include a wire bonding process by which a semiconductor chip is mounted on a lead frame or a substrate. The circuit includes a lead formed on the outside the chip or an electrode pad formed on the substrate are connected by a metallic wire such as aluminum or a gold.
Typically in this wire bonding process, the metallic wire is drawn out to a bonding tool section of a wire bonder (wire bonding apparatus) and welded with pressure to a bonding pad of the semiconductor chip. In an ultrasonic type wire bonder, an ultrasonic vibration is applied to the bonding tool. The wire is bonded by pressing both it and the tool against the chip bonding pad.
When a metallic wire is bonded to a semiconductor chip in a conventional wire bonder, a duration of bonding time is preset. Then an ultrasonic wave and a pressing force are applied to the tool and wire for the preset bonding time. The preset time is determined by taking account of the bonding characteristic of the bonding section to assure maximum dispersion of the wire and adding a certain time margin for safety.
However, when a metallic wire is actually welded in this the length of time required for completion of bonding can vary from the preset time depending on dispersion of the welding characteristic of the individual pad. Thus even though a good bond is established the bonding operation continues until the preset time has elapsed. Therefore, unnecessary vibration and pressing force is exerted on the semiconductor chip, causing cracking or breakage, and deterioration of the internal function.
As an example of this problem is present in a power control unit or the like, in particular, in rotating equipment such as a motor for a motor-driven vehicle or other motors or a generator. In order to shorten the length of the wiring to decrease resistance to improve the performance of semiconductor devices and to achieve higher efficiency of production processes as well as higher installation density, semiconductor devices are mounted as bare chips on an electrode pattern or a circuit pattern of a substrate. When bonding is performed for such bare chips by the aforenoted conventional manner, the problem of unnecessary vibration and pressing force is amplified. Thus increasing the problem of the cracking and deterioration.
Therefore it is an object of this invention to provide a wire bonding method and apparatus capable of bonding a metallic wire to a semiconductor chip in an optimum and a minimum time regardless of the welding characteristic of the bonding section.